In color imaging separate images in three primary colors and, generally, also a separate image in black are combined on a single substrate in registration. This invention is directed to such imaging systems in which toner images are combined on an intermediate member prior to being transferred as a unit to paper or other final substrate. Such transfer previously has been accomplished by heating at the transfer station and by preheating immediately prior to the transfer station. Heating at the transfer station is effective provided the high temperature levels employed are acceptable with the overall design of the printer and consistent operation can be achieved. Although materials are known, such as silicone rubbers, which function extremely well as an intermediate surface, the ability of the material to release toner degrades with usage to such a degree that the quality of the image is noticeably affected. As the intermediate surface must be large enough to contain the entire image (in the case of a drum, a circumference of at least 14 inches is typically required), the machine component is necessarily bulky and expensive, and therefore not readily designed as a replaceable supply item.
In addition to possessing excellent release properties, the intermediate surface must also be compatible with electrostatic transfer of multiple layers of toner from a photoconductor. The electrical properties constraints imposed by this requirement further limit the choices of material. No material is known which meets all of the transfer requirements without preheating and is durable enough to withstand more than 100,000 image releases to paper.
The temperature requirements for transfer of toner from the intermediate surface to paper are such that the transfer roller temperature must be in excess of 160 degrees C. to effect 100 percent transfer of toner to paper from the best known intermediate drum release materials. As the release properties of materials degrade with usage and with contact against paper, the transfer roller temperature requirements increase. As a result of the high transfer roller temperature, the temperature of the intermediate transfer surface increases during long printing runs, in particular when only one primary color is used and the transfer roller is engaged for a large proportion of the operating cycle of the imaging device. Without elaborate cooling schemes, the temperature of the photoconductor, due to continuous rolling contact against the intermediate surface, is increased beyond the range of acceptable operation. This condition is exaggerated in the case of a machine operating in a hot environment. In addition, it is necessary to prevent excessive contact between the hot transfer roller and the intermediate surface, not only to prevent overheating of the photoconductor, but to prevent excessive vaporization of the process carrier fluid, such as mineral oil, of a liquid toner. This constraint precludes the use of print media substantially narrower than the transfer roller width, effectively limiting the imaging operation to one width of paper.
Heating prior to the transfer tends to coalesce the toned image and reduce or eliminate the need to heat at actual transfer, but effective preheating of color images has not been previously accomplished. This invention accomplishes such preheating, thereby avoiding the foregoing constraints. Preheating is shown in the following prior art references, but none employ contact heating with electrical bias as does this invention. Contact heating is employed in U.S. Pat. No. 5,247,334 to Miyakawa et al. The following employ radiant heating as preheating for transfer: U.S. Pat. Nos. 5,158,846 to Bujese, No. 4,992,833 to Derimiggio and No. 4,453,820 to Suzuki.